Photosensitive compositions comprising silver complexes of 3-substituted-4-thiazoline-2-thiones and photosensitive elements comprising such compositions

ABSTRACT

Complex silver salts, formed by silver ion with a 3-substituted4-thiazoline-2-thione which has an active hydrogen in the group substituted at the 3-position, are found to be photosensitive, i.e., capable of forming photographic images by photoexposure and development, useful for making photographic films, plates and the like.

United States Patent Grant M. Heist;

Wilbert J. Humphlett; James R. King, all 01 Rochester, N.Y.

Oct. 1, 1968 Nov. 16, 1971 Eastman Kodak Company Rochester, N.Y.

Inventors Appl. No. Filed Patented Assignee PHOTOSENSITIV E COMPOSITIONS COMPRISING SILVER COMPLEXES OF S-SUBSTITU'I'EM THIAZOLINE-Z-TI'IIONES AND PHOTOSENSITIVE ELEMENTS COMPRISING SUCl-I COMPOSITIONS 23 Claims, No Drawings U.S. C1 96/94 R, 96/88 Int. Cl. G030 1/02 Field of Search 96/94, 88, 65, 64, I 14.6, 50; 117/34 [56] References Cited UNITED STATES PATENTS 3,152,904 10/1964 Sorensen et a1 96/76 3,279,920 10/1966 Theodorou 96/64 X Primary Examiner-William D. Martin Assistant Examiner-Theodore G. Davis AnorneysWil1iam H. J. Kline, Bernard D. Wiese and Gerald E, Battist PHOTOSENSITIVE COMPOSITIONS COMPRISING SILVER COMPLEEES OF 3-SUBS'ITI'UTED-4- THIAZOWE-Z-THHONIES AND PHOTOSENSITIVE ELEMENTS COWRlSING SUCH COMPOSITIONS This invention relates to photography and particularly to novel photosensitive silver compositions and to photographic elements comprising such compositions.

Photosensitive silver halides have been the basis of silver photography for many years. By photoexposure, the silver halides are made to be differentially reducible by photographic developers. Silver halides, as conventionally used in silver halide photographic emulsions, are practically insoluble in water and in conventional photographic developer solutions; it is necessary after exposure and development of a photograph made with photographic silver halide emulsions to stabilize the photography by removing undeveloped silver halide by means of a silver halide solvent.

Among the objects of the present invention are to provide novel photosensitive compositions and to provide photographic elements comprising such compositions, as described more particularly herein. In preferred embodiments, fixing by means of a silver halide solvent is not necessary.

An essential photosensitive image forming component of a photographic composition according to the present invention is a complex fonned by silver ion in solution with a 3-substituted-4-thiazoline-2-thione derivative having the formula;

wherein R is an aliphatic organic radical having a terminal acidic hydrogen atom bonded to an oxygen atom (e.g., carboxyalkyl, sulfoalkyl, hydroxyalkyl, and the like) preferably containing one to 12 carbon atoms, and the salts thereof; and each of R and R represents a member selected from the group consisting of hydrogen and organic radicals. Some especially preferred specific compounds corresponding to the above formula include 1. 3-Carboxymethyl-4-methyl-4-thiazoline-Z-thione; 2. 3-(2-Carboxyethyl)-4-methyl-4-thiazoline-2-thione; 3. 3-( l ,Z-Dicarboxyethyl)-4-methyl-4-thiazoline-2- thione;

4-methyl-3-(2-sulfoethyl)-4-thiazoline-2-thione potassium salt; 5. 3-(l-carboxyethyl)-4-methyl'4-thiazoline-2-thione 6. 3-( l-Carboxy-3-methylthiopropyl)-4-methyl-4- thiazoline-Z-thione; 7. 3-(Z-Carboxyethyl)-4-hydroxymethyl-4-thiazoline-2- thione; 8. 4-Carbosy-3-( l-carboxyethyl)-4-thiazoline-2thione;

and 9. 3-(Z-Hydroxyethyl)-4-methyl-4-thiazoline-2-thione. See J. Org. Chem.,29, 2146 [964); J. Chem. 500.,1955, 2990.

Synthesis of 4-thiazoline-2-thiones bearing a carboxy group in the 3-position has been described in Berichte, 56, 289' (i923); Compl. Rendus, 22, 211 (1938), C.A. 32, 6246- (1938); Joum. Am. Chem. 800., 72, 1879 (1950). These syntheses involve treating a dithiocarbamic acid derived from an amino acid and carbon disulfide with an a-halogenated ketone. We have slightly modified the earlier procedure by employing methanol instead of water as solvent and by using two equivalents of amino acid and carbon disulfide per equivalent of a-halo ketones. In some instances, depending on reaction conditions, an intermediate reaction product, either a 4-hydroxythiazolidine-Z-thione or a dithiocarbamate is obtained which can then be dehydrated or cyclodehydrated to the 3-substituted-4-thiazoline thione. We have prepared a variety of 3-substituted-4thiazoline-2thiones using amino acids such as glycine; DL- or B-alanine; 4-aminobutyric acid; DL-aspartic acid; taurine; DL-methionine; 6aminohexanoic acid and m-aminobenzoic acid and employing such a-halo ketones as chloroacetone; phenacyl bromide; 3-chloro-2butanone; ethyl brornopyruvate; ethyl 2- and 4chloroacetates; lacetoxy-3-chloro-2propanone; l-butysulfonyl- B-bromo- 2- propanone and 3-chloro-2,4-pentanedione.

The following syntheses typify preferred procedures for preparing certain 3-substituted-4-thiazoline-2-thiones:

A 4Acetoxymethyl-3-(ZcarboxyethyUA-thiazoline-Z- thione A mixture of 55.2 grams (0.62 mole) of D,L-alanine and 36.8 grams (0.56 mole) of potassium hydroxide in 150 milliliters of methanol is made with warming and stirring. The small amount of D,l..-alanine in excess fails to dissolve. To the stirred mixture cooled in an ice bath is added 34 milliliters (0.56 mole) of carbon disulfide over 20 minutes, forming a gummy, white solid. After the cooled mixture is stirred for one hour, a solution of 42 grams (0.28 mole) of l-acetoxy-3- chloro-Z-propanone in an equal volume of methanol is added, with cooling and stirring, over 20 minutes. The reaction mixture is stirred for 1 hour, with cooling. The reactions are exothermic. Sufficient water, about 200 milliliters, to form a solution is added, and the solution is stirred 15 minutes longer. Methanol is removed in vacuo, leaving a yellow solution. Acidification with excess concentrated hydrochloric acid causes an oil to separate which solidifies on standing overnight. Filtration yields 66 grams of crude product, m.p. 144. Pure product is obtained after two recrystallizations, amounting to 46 grams of light tan needles.

B. 3-(2-Carboxyethy])-4-phenyl-4-thiazoline-2-thione A solution of 10 grams (0.035 mole) of 3-( 2carboxyethyl)-4 -hydroxy-4-phenylthiazolidine-2-thione in 50 milliliters of glacial acetic acid is refluxed for 0.5 hour. On cooling, crystals form as prisms. Water milliliters) is added, and the crystals are collected, giving 7 grams of the thiazoline, m.p. l34-l36. A second crop gives 2.5 grams.

C. 3-(2-HydroxyethyD-4 -methyl-4-thiazoline-2-thione A solution of 61 grams (1 mole) of Z-aminoethanol and 98.1 grams (1 mole) of potassium acetate is made in 600 milliliters of methanol with warming and stirring. To the stirred solution cooled in an icebath is added 60.3 milliliters (1 mole) of carbon disulfide over 20 minutes. Continued stirring for 2% hours results in the formation of a white precipitate. A solution of 40 milliliters (0.5 mole) of chloroacetone in an equal volume of methanol is added over 20 minutes to the cooled, stirred mixture and then stirring is continued for 1 hour. Sufficient water is added to form a solution, and methanol is removed in vacuo leaving an oil, which resists crystallization. The oil is collected by extraction with ether. The combined ether phases are extracted with water and dried over anhydrous sodium sulfate. Ether is distilled off and the final traces of solvent removed in vacuo. The resultant oil readily crystallizes yielding 56 grams of product, mp. 65". Recrystallization from water gives product of unchanged melting point.

Table 1 describes other 3-substituted-4-thiazoline-2thiones derived from amino acids by such procedures:

TABLE I 4-thiazoline-2-thiones derived from amino acids S Rr-lcli $=S R2- C---N-R1 M.P., Yield, R1 R2 R Fonnula 0.0) percent CHzCOzH CHQCOZCZHE H CoHnNOrSz 142-143 62 CHzOOzH CHzCOzH H C1H1NO4S2 175-176 100 CHzCOzH CHzO COCH; H CsHsNOASz 162-163 44 CHzCOzH CH3 COzCzHs CsHnNO4S2 159-160 83 CHsCOzH CH: 2H C'IH1NO4S2 224-225 100 CHaCOzH CH3 COCHa CsHqNOsSz 4-185 83 (CH2 200211 H H CflH'INO2S2-H2O 58-60 71 (CH: tCOzH OH: H C'IHONO-QS? 132-134 95 (CH: sCOtH CHaOCOCI-Ia H CsHttNOiSz 153 86 (0112100211 CHzOH H CzHsNOaS: 141 84 2 200211 CHzSOzCiHq H CnHnNOsSa 137-138 74 1 9200211 (35115 H CtzHirNOzSz 5 136-137 100 (CHzMCOaH CH; COCH: COHIINO3S2 142-143 92 CH2; aCOzH CH: COnCzHr CmHuNOASz 145-146 48 (CH2 2002B OH; 00211 CsHsNOtSz 206-207 83 H (011:; (30211 H H CaHINOsS: 146-148 38 CH(CHa COsH COzCzHs H COHHNOIS2 194 73 CH(CH;; COzH COIH H C7H1NO4$2 212 88 CH(CHs COzH CHnCOzCzHs H Ci0H14NO4S2 60-61 42 CH(CHa)COsH HzCOzK H CsHsKNOtSz 232 56 CH(CH3)CO:H a COCH: CsHnNOsS: 136-137 91 (CHzgaCOzH CH-CO2CH5 H CflfllsNotsz 100-101 79 s aCOzH CHsCOzH H CsHnNOiSa 147-148 100 (CH7)3 02 (.HsSOzCtHs H nHtsNOtSa 1 60 (CH2)3C02H C5115 H CuHuNOzSz 137-138 52 (CH2)3C02H CH3 COzCzHs CnHuNOsSz 122-123 71 (CHfiaCOzH CH3 2H CIH11N04S2 203 89 (CH2) 5002K CH3 CmHuNOzSz 86-87 70 CHCOzH CH a H C3HDNO|S2 217-218 26 CHrCOzH CHzCHgSOaK CH H CgHsNOaSgK 362 CH2S(CH2)2CHCO:H OH; H CaNnNOzS: 130 80 CsHACOzH-m CH H CnHsNOzSz b 191-193 66 I Recrystallized from water unless otherwise indicated. b From 50% ethanol.

s From methanol.

Preparation of a photosensitive composition according to the invention can be done by preparing an aqueous solution containing the photoactive silver complex simply by dissolving a water-soluble silver salt in aqueous solution with a near equimolar amount of a 3-substituted-4-thiazoline-2-thione selected from the class described above. Molar proportions of the silver salt and the 4-thiazoline-2-thione compound preferably should be kept about 1:1 in the solution although some variation from this ratio usually will be tolerable without seriously reducing photoactivity of the composition.

The silver complex can be mixed with gelatin or with other hydrophilic colloids such as polyvinyl alcohol, synthetic gelatin substitutes and the like, or a mixture thereof, to make a photographic emulsion for coating on a photographic support such as film, plates, paper or the like.

Following are specific examples describing in detail our most preferred mode for carrying out the invention.

EXAMPLE 1 Three separate solution parts are prepared.

Part A: To 20 milliliters distilled water add 0.62 gram of 3- l,-dicarboxyethyl)-4-methyl-4-thiazoline-2-thione and heat until a clear solution is obtained.

Part B: Dissolve 0.65 gram of p-toluene sulfonic acid, silver salt in 20 milliliters distilled water.

Part C: Milliliters of 10 percent gelatin solution.

Pour Parts A and B simultaneously and slowly with continuous stirring into Part C. Add a few drops of saponin, filter the solution and then coat on conventional photographic paper support using a coating block with a coating knife set at 0.006 inch. After the coating has dried, the coated sample is exposed photographically on an Eastman lB sensitometer through a step tablet for 30 seconds at positive film setting, The exposed sample is developed for 30 seconds at 75 F. in Kodak Developer D-76 diluted lzl with water then put through a ry. A stepped silver image with low minimum density is obtained.

EXAMPLE 2 with the emulsion as in example 1 and tested by the same procedure described in example i to produce a stepped silver image of good contrast and density.

Emulsions similar to those described in the above examples are prepared but using other 4-thiazoline-2-thiones that do have the acidic hydrogen-containing radical at the 3-position. None are found that have any significant photosensitivity. Similar emulsions made with the other 3-substituted-4- thiazoline-Z-thiones named in the numbered list above and with those others described in table I are found to be photosensitive to varying extents but all comparable with the preferred examples above.

The reaction between the soluble silver salt and the 4- thiazoline-Z-thione is thought to be formation of a complex ion involving interaction of the silver ion and the active radical at the 3-position of the 4-thiazoline-2-thione, resulting in the silver salt complex. We have not yet definitely established the exact nature of the complex that is formed.

in the foregoing examples, we have described in detail only some preferred embodiments of the invention for the purpose of setting out our most preferred mode of carrying out the invention. Other embodiments can involve variations from the specific examples described. Other compounds of the formula described above can be substituted for the specific 4- thiazoline-Z-thione compounds used in the examples. The invention permits considerable variation in the methods that can be used for preparing the light-sensitive coatings. Other components such as other soluble silver salts instead of the toluene-sulfonie acid silver salt, can be used or hydrophilie colloids other than gelatin can be substituted.

in defining the above formula, we have mentioned that there must be attached at the 3-position on the ring a group which contains an active hydrogen atom. This is necessary for photosensitivity of the silver complex. Also, it is necessary to photosensitivity that the thione sulfur atom must not be converted to a mercapto form. Compounds having acid groups at R or R positions can be used but, again, they are effective only if the R, radical has the necessary active hydrogen atom. Compounds in table I are specific examples of compounds which the defined formula, useful for the invention.

ln defining the above formula, we have mentioned that each of R, and R can represent hydrogen or an organic radical. In table I we have described several preferred ones of such organic substituents, but these positions have not seemed critical to the photosensitive complex formation and we would include within our invention any 4-thiazoline-2-thione compound having the critical structure as defined and that forms a photosensitive complex with silver as described, regardless, of the nature of the R, and R organic radical substituents. 1

With the foregoing statement in mind, we can mention that the R, and R, substituents in the formula can preferably include hydrogen, lower aliphatic radicals containing one to carbon atoms, including radicals of lower aliphatic hydrocarbons, acids, esters, salts, ketones and the like, or can include aryl radicals, mixed aryl-alkyl radicals and the like. Substituents at these positions can affect solubility of the photosensitive complex and in preferred embodiments .the eomplexing agents can be selected for their better solubility characteristics, but our invention includes all embodiments that form the photosensitive complex. 1

Typical aliphatic groups having a terminal acidic hydrogen atom, which can constitute the R, substituent, can include carboxyaliphatic, carboxythiaaliphatic and dicarboxyaliphatic, particularly carboxyalkyl and dicarboxyalkyl and including carboxythiaalkyl; sulfoaliphatic and hydroxy aliphatic such as sulfoalkyl and hydroxyalkyl. In most such groups comprising alkyl or thiaalkyl groups, the alkyl will preferably be alkyl containing one to six carbon atoms.

Photosensitive elements comprising the silver salt complexes of the invention as essential photosensitive components can be in a variety of fonns. The elements can comprise any suitable support. Typical supports can be paper or film sheets, glass or metal plates and the like, for example, baryta coated or plain paper supports or papers coated with a resin coat such as polyethylene or other polyolefin. The support can be a film support such as cellulose esters or polyester films, particularly the cellulose acetates or polyethylene terephthalate films. The elements can comprise one, or more than one, photosensitiverecording layer and in multiple layer elements the elements can comprise photosensitive layers of other compositions in combination with a separate recording layer of a silver salt complex described herein. Elements comprising one or more recording layers of the silver salt complexes can be made for various uses such as color photography, X-ray recording, ultraviolet recording and the like. The recording layer comprising the silver salt complex can be coated without a binder for some applications or can be coated in a layer comprising any suitable binder, including particularly hydrophilic colloids such as gelatin, polyvinyl alcohol and other synthetic polymer hydrophilic colloids. The silver salt complexes can be sensitized to improve photographic speed with chemical sensitizers and can be sensitized with spectral sensitizing dyes. Generally, chemical and spectral sensitizers known for use in silver halide photographic emulsions can be included, in recording layers containing the silver salt complexes of this invention. Layers comprising the silver salt complexes can be the photosensitive layer or a diffusion transfer photographic element, and in some embodiments can be used in diffusion transfer processes without need for a specific silver salt solvent, as many of the silver salt complexes will be soluble enough in the developer solution without need for a specific solvent. The silver salt complexes can be developed known for use with silver halides, employing the conventional developing agents. When desired, aphysical developer may be employed to intensify the developed silver image.

The silver salt complexes are useful as the photosensitive silver salt in a dry development system in which an image is developed by a dry physical development process. An incorporated silver salt such as silver behenate, in presence of a reducing agent is capable of intensifying the image by a dry heat process. A process of this kind, but using different photosensitive silver salts, is described in U.S. Pat. No. 3,152,904 patented Oct. 13, 1964 to David P. Sorensen et al.

Photosensitive emulsions comprising the silver salt complexes can be coated by conventional coating techniques that are employed for coating silver halide emulsions. The emulsions can contain adjuvants employed in various silver halide emulsions such as conventional wetting agents, hardeners, antifoggants, dyes, dye precursors in color imaging layers, and the like.

Although the invention has been described in detail with particular reference to preferred embodiments thereof, variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.

We claim:

1. A photographic element comprising a support and coated thereon a photosensitive recording layer comprising, as an essential photosensitive component thereof, a silver salt complex of a 3-substituted-4-thiazoline-2-thione compound having the formula:

wherein R, is an aliphatic radical containing from one to 12 carbon atoms which has a terminal acidic hydrogen atom bonded to an oxygen atom and salts thereof, and R, and R, respectively are each selected from the group consisting of hydrogen, aliphatic radicals or a phenyl radical.

2. A photographic recording element defined by claim 1 wherein R, is a carboxy aliphatic radical.

3. A photographic recording element defined by claim 1 wherein R, is a carboxy alkyl radical.

4. A photographic recording element defined by claim 1 wherein R, is carboxymethyl.

5. A photographic recording element defined by claim 1 wherein R, is carboxyethyl.

6. A photographic recording element defined by claim 1 wherein R, is a dicarboxyaliphatic radical.

7. A photographic recording element defined by claim 1 wherein R, is a dicarboxyalkyl radical.

8. A photographic recording element defined by claim 1 wherein R, is diearboxyethyl.

9. A photographic recording element defined by claim 1 wherein R, is a sulfoaliphatic radical.

10. A photographic recording element defined by claim 1 wherein R, is a sulfoalkyl radical.

11. A photographic recording element defined by claim 1 wherein R, is sulfoethyl.

12. A photographic recording element defined by claim 1 wherein R, is a carboxy thia-aliphatic radical.

13. A photographic recording element defined by claim 1 wherein R, is a carboxy thia-alkyl radical.

14. A photographic recording element defined by claim 1 wherein R, is carboxythiapentane.

15. A photographic recording element defined by claim 1 wherein R, is a hydroxyaliphatic radical.

16. A photographic recording element defined by claim 1 wherein R, is a hydrdxyalkyl radical.

17. A photographic recording element defined by claim wherein R, is hydroxyethyl.

wherein R is an aliphatic radical containing from one to 12 carbon atoms which has a terminal acidic hydrogen atom bonded to an oxygen atom and salts thereof, and R, and R,

respectively are each selected from the group consisting of hydrogen, aliphatic radicals or a phenyl radical, as an essential photosensitive component of said emulsion.

20. A photographic emulsion according to claim 19, wherein R is a carboxy aliphatic radical and R, and R, are each hydrogen atoms, lower aliphatic radicals containing one to 10 carbon atoms or phenyl.

21. A photographic emulsion comprising a silver salt complex of 3-(1,Z-dicarboxyethyl)-4-methyl-4-thiazoline-2-thione as an essential photosensitive component of said emulsion.

22. A photographic emulsion comprising a silver salt complex of 3-( l-carboxyethyl)-4-methyl-4-thiazoline-2-thionc as an essential photosensitve component of said emulsion.

23. A photographic emulsion according to claim 19 wherein the aliphatic radical is a lower alkyl radical, an acid radical or salt thereof, an ester radical, a ketone radical, or an aryl or a mixed aryl-alkyl radical.

# i t :I t

@2 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,620, 7119 Dated November 16, 1 971 Inventor-(s) Grant M. Haist, Wilbert J. Humplett, James R. King It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 1 line 1 7, "photography" should read ---photograph---.

Col. 1 line 55, "L -carbosy should read Lp-carboxy Col. 2, line 9, "3-substituted-h-thiazoline-Zthiones" should read 3-substituted-Lp-thiazoline-Z-thiones Col. 2, line 1 I "6minohe'xanoic" should read -aminohexanoi Col. 2, line 1 9, "Mcetoxymethyl-B- (Zoarboxyethyl) should read h-Aoetoxymethyl-B-(Z-carboxyethyl) Col 2, line M2, "3 (z u should read 3 (2-oerboxyethyl) Col. 3, line 1 "-Zthiones" should read 2-thiones Col. 3, line 62, "(1,-dicarboxyethyl)" should read (I ,Z-dicarboxyethyl) Col. 5, line 1L 'whioh the defined formula, should read ---within the defined formula--- Claim 1 9, line 2, "B-substituted-H-thiazoline" should read 3-substituted-Ip-thiazoline Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Ettesting Officer Commissioner of Patents 

2. A photographic recording element defined by claim 1 wherein R1 is a carboxy aliphatic radical.
 3. A photographic recording element defined by claim 1 wherein R1 is a carboxy alkyl radical.
 4. A photographic recording element defined by claim 1 wherein R1 is carboxymethyl.
 5. A photographic recording element defined by claim 1 wherein R1 is carboxyethyl.
 6. A photographic recording element defined by claim 1 wherein R1 is a dicarboxyaliphatic radical.
 7. A photographic recording element defined by claim 1 wherein R1 is a dicarboxyalkyl radical.
 8. A photographic recording element defined by claim 1 wherein R1 is dicarboxyethyl.
 9. A photographic recording element defined by claim 1 wherein R1 is a sulfoaliphatic radical.
 10. A photographic recording element defined by claim 1 wherein R1 is a sulfoalkyl radical.
 11. A photographic recording element defined by claim 1 wherein R1 is sulfoethyl.
 12. A photographic recording element defined by claim 1 wherein R1 is a carboxy thia-aliphatic radical.
 13. A photographic recording element defined by claim 1 wherein R1 is a carboxy thia-alkyl radical.
 14. A photographic recording element defined by claim 1 wherein R1 is carboxythiapentane.
 15. A photographic recording element defined by claim 1 wherein R1 is a hydroxyaliphatic radical.
 16. A photographic recording element defined by claim 1 wherein R1 is a hydroxyalkyl radical.
 17. A photographic recording element defined by claim 1 wherein R1 is hydroxyethyl.
 18. A photographic recording element defined by claim 1 wherein R2 and R3 can each be hydrogen atoms, aliphatic radicals containing from one to 10 carbon atoms or phenyl.
 19. A photographic emulsion comprising a silver salt complex of a 3-substituted-4-thiazoline-2-thione having the formula: wherein R1 is an aliphatic radical containing from one to 12 carbon atoms which has a terminal acidic hydrogen atom bonded to an oxygen atom and salts thereof, and R2 and R3 respectively are each selected from the group consisting of hydrogen, aliphatic radicals or a phenyl radical, as an essential photosensitive component of said emulsion.
 20. A photographic emulsion according to claim 19, wherein R1 is a carboxy aliphatic radical and R2 and R3 are each hydrogen atoms, lower aliphatic radicals containing one to 10 carbon atoms or phenyl.
 21. A photographic emulsion comprising a silver salt complex of 3-(1,2-dicarboxyethyl)-4-methyl-4-thiazoline-2-thione as an essential photosensitive component of said emulsion.
 22. A photographic emulsion comprising a silver salt complex of 3-(1-carboxyethyl)-4-methyl-4-thiazoline-2-thione as an essential photosensitve component of said emulsion.
 23. A photographic emulsion according to claim 19 wherein the aliphatic radical is a lower alkyl radical, an acid radical or salt thereof, an ester radical, a ketone radical, or an aryl or a mixed aryl-alkyl radical. 